Duplexers that divide transmit signals and receive signals are widely used in conventional mobile communications equipment, typified by mobile telephones. A duplexer basically includes a transmit filter connected between the antenna terminal and transmit terminal, and a splitter circuit and receive filter connected between the antenna terminal and receive terminal in series. The splitter circuit contained in the duplexer is used to isolate the effects that the transmit/receive signals have on the circuits of the other party, and thus the transmit signals sent from the transmit terminal are sent to the antenna terminal without reaching the receive terminal, and the receive signals sent from the antenna terminal are supplied to the receive terminal without reaching the transmit terminal.
A splitter circuit used in a duplexer typically includes an ordinary LC circuit and a quarter-wavelength phaseline. (See Japanese Patent Application Laid Open No. 2001-24476.)
However, if the splitter circuit used in a duplexer is composed of an LC circuit and quarter-wavelength phaseline, this results in a problem wherein it impedes attempts to make the duplexer more compact. Namely, in a splitter circuit that uses an LC circuit, the inductor (L element) and capacitor (C element) are of a large size, so a plurality of electrode pads are required for mounting and thus it is intrinsically difficult to make the circuit more compact. In addition, in a splitter circuit that uses a quarter-wavelength phaseline, when a substrate with a dielectric constant of approximately 7, for example, is used, a phaseline of approximately 10 mm is required in the 2 GHz band, so the size is considerably large in comparison with transmit filters or receive filters that can be implemented with a size of roughly 1-1.5 mm square, respectively, in the same frequency band.
As is apparent from foregoing, in order to make the duplexer more compact, it is necessary to make the splitter circuit contained therein more compact.